1. Field of the Invention
This invention relates to a turbocharger for an internal combustion engine. More particularly, this invention relates to a turbocharger with variable turbine geometry having guide vanes with features to control exhaust gas flow.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's horsepower without significantly increasing engine weight.
Thus, turbochargers permit the use of smaller engines that develop the same amount of horsepower as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of a cleaner environment.
Turbochargers typically include a turbine housing connected to the engine's exhaust manifold, a center bearing housing, and a compressor housing connected to the engine's intake manifold. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft rotatably supported in the center bearing housing connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. As the compressor impeller rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the engine's cylinders via the engine's intake manifold.
It is often advantageous to regulate the exhaust gas flowing to the turbine wheel to improve efficiency, responsiveness or the operating range of the turbocharger. One method of regulating the exhaust gas flowing to the turbine wheel is commonly referred to by several names, including Variable Turbine Geometry (VTG), Variable Geometry Turbine (VGT), Variable Nozzle Turbine (VNT), or simply Variable Geometry (VG). VTG turbochargers include a plurality of adjustable guide vanes pivotally supported by a vane support ring within a wheel inlet leading to the turbine wheel. The space between adjacent guide vanes constitutes flow channels for the exhaust gas flowing to the turbine wheel and the geometry of the flow channels is adjustable by adjusting the guide vanes within a pre-determined range of angular positions between an open position and a closed position. In order to provide a high boost pressure at low engine speeds, the guide vanes are adjusted to constrict the flow channels between adjacent guide vanes. This results in the exhaust gas moving through the flow channels at a high speed. The increased kinetic energy of the exhaust gas is transferred to the turbine wheel, increasing the boost pressure. At high engine speeds, the guide vanes are adjusted to open up the flow channels between adjacent guide vanes. This results in the exhaust gas impacting the turbine wheel at a lower speed, thus decreasing the boost pressure.
Spacing between the vane support ring and a ring-shaped wall of the turbine housing, in conjunction with the width of the guide vanes, is critical to prevent leakage of the exhaust gas around edges of the guide vanes. Clearance is provided between the edges of the guide vanes and the vane support ring and the ring-shaped wall to permit pivotal movement of the guide vanes and to allow for thermal expansion due to the hot exhaust gas. When the guide vanes are in the closed position, such clearance will lead to a problem of leakage of the exhaust gas around the edges of the guide vanes. Similarly, when the guide vanes are opened, such clearance also results in leakage of the exhaust gas around the edges of the guide vanes, rather than directing all the exhaust gas through the flow channels, thereby reducing the efficiency of the turbocharger.
In addition, when the guide vanes are adjusted to the open position or an angular position between the open and closed positions, the exhaust gas is guided through the flow channels toward the turbine wheel by opposing surfaces of adjacent guide vanes. As the exhaust gas flows between adjacent guide vanes, high and low pressure regions will lead to swirling and transverse flow of the exhaust gas rather than smooth flow of the exhaust gas along the length of the guide vanes.
It is desirable therefore to provide a turbocharger with guide vanes that prevent or minimize leakage of the exhaust gas. It is also desirable to provide guide vanes that prevent or minimize swirling and transverse flow of the exhaust gas within flow channels between adjacent guide vanes.